I agree with Victor.  When code launches a process with subprocess APIs, it is expected that the subprocess module manages the process.

Calling os.waitpid directly instead of using subprocess's APIs breaks that expectation.

Also, WNOWAIT and waitid() (not waitpid()) are not available on all platforms.  Nor is pidfd_open().  Code using any of those for such a PR needs to be conditional on their runtime availability.